Starch, a natural polymer (C.sub.6 H.sub.10 O.sub.5).sub.n derived from plant materials, is commonly found in the form of tiny microscopic granules (5-25 microns in diameter) comprised of stratified layers of starch molecules formed around a hilum nucleus. The starch granule shape may be round, oval or angular, and consists of a radially oriented crystalline aggregate of two anhydrous D-glucose polymers: amylose and amylopectin. The former is a straight chain polymer of several hundred glucose units linked by alpha-1-4-glycosidic linkages. Amylopectin is a branched polymer of several thousand glucose units with alpha-1-6-glycosidic linkages at the branched points and alpha-1-4 linkages in the linear regions. Individual branches may have between 20-30 glucose residues.
Heretofore, physical and chemical modification of starch has been accomplished through a variety of processes in order to realize improved performance characteristics in the food, paper, textile and pharmaceutical industries. For example, improved rheological and thermal stability properties of modified starches result in superior frozen, instant, dehydrated, and heat and serve food products which would not otherwise be economically competitive. Similarly, in the pharmaceutical industry, solubility and hardness properties of starch excipients are improved to effect the appearance and performance of tablets. Other physical properties of starches important to formulators and food processors today include viscosity, acid stability, moisture resistance or affinity, gel characteristics, decomposition rates (shelf life), shear resistance, texture, mouth feel and clarity when in solution.
Modification of starch is conventionally accomplished through thermal or chemical means. Pregelatinization, for example, is the pre-cooking of starch and starch products to make them cold water soluble. This solubility property, more commonly referred to as "cold water swelling", finds utility in instant foods such as puddings, pie fillings, analogues, and textured products. Pre-cooking is accomplished by application of a starch slurry to a steam-heated roll where the starch is cooked instantly and dried. Thus, pregelatinized starches are those which have been hydrated and treated thermally. Through moisture and heat treatments the crystallinity of several starch products can be modified to provide desired property changes.
Starch may also be modified chemically by a process known as crosslinking to provide inhibition properties. Bi- or polyfunctional reagents such as phosphorus oxychloride are used to covalently bridge, or crosslink, two starch molecules at various locations along their chains to provide viscosity stability as well as acid, heat and shear tolerance. These properties find utility in acid foods (salad dressing and pie fillings), canned foods, gravies and sauces, cream-style corn, and oriental foods.
Another chemical modification process of starch is substitution. Here, the introduction of substituent groups on starch by treatment with monofunctional reagents which react with the hydroxyl groups on starch produces starch esters and starch ethers. Treating substituents include acetate, succinate, phosphate, hydroxypropyl, and octenylsuccinate. The primary purpose of substitution is to impart resistance to retrogradation and gelling of amylose and eliminate the association of the linear segments of amylopectin at low temperature. Other effects of this process include a lowering of gelatinization temperature, increased viscosity, improved colloidal properties, and modification of hydrophilic or hydrophobic character.
Still other chemical modification processes such as acid hydrolysis and oxidation may be employed to impart low hot paste viscosity, high alkali number, a high ratio of cold to hot paste viscosity, color changes, and high adhesive and binding powers to the starch composition. Starches so chemically modified find utility in the manufacture of gum candy and as coating agents for confections and breaded foods.
Each of the above modification processes, however, suffer from various shortcomings and result in starch products with physical property limitations. In the field of pharmaceuticals starch is frequently used as a binder for active ingredients and as a disintegrant upon contact with water or gastric solutions. Pregelatinized corn starch provides tablets with hardness properties in the range of 1 to 4 Kp. Present demands, however, require hardness levels in the range of 10-14 Kp, an expectation which starch modified by prior art methods simply can not meet. While the use of starch in tableting formulations is still common practice, problems of uniformity between modified batches and a demand for tablets of greater hardness resulted in its departure from the status of a preferred pharmaceutical excipient.
Prior art methods of chemical starch modification are also inherently cost in-effective requiring the additional expense of crosslinking chemicals or functional reagents to produce the desired physical characteristics in the substrate. Disposal problems associated with unwanted reaction by-products further adds to cost and environmental concerns. Also, chemical modification methods yield product in batch quantities, rather than on a continuous or semi-continuous basis and, therefore, are less time efficient. Production rates are further diminished when more than one chemical modification must be made to the starch substrate to yield a product with all of the desired characteristics. Moreover, the starch end-product itself often suffers from other limitations similar to the deficient tablet hardness profiles, discussed above. Inferior viscosity, shear resistance and thermal profiles of the starch end-product, for example, may frustrate the performance of products incorporating starch modified by prior art means.
It is clear that a need exists for a method of modifying the physical properties of starch and other polymer compositions to provide improved performance characteristics useful in the food, pharmaceutical and other industries. The subject invention completely obviates all of the above described shortcomings by providing a method and apparatus for the physical modification of polymers such as starch by pressure processing techniques; the modified novel end-products themselves being characterized by improved physical properties.